Voltage Of Solar Cells Research Articles

Back Contact Optimization for Sb<sub>2</sub>Se<sub>3</sub> Solar Cells

Antimony selenide (Sb<sub>2</sub>Se<sub>3</sub>) has advantages of low-toxicity, abundant and excellent photoelectric properties. It is widely considered as one of the most promising light-harvesting materials for thin-film solar cells. However, the power conversion efficiency of the Sb<sub>2</sub>Se<sub>3</sub> thin-film solar cell is still far inferior to that of cadmium telluride, copper indium gallium selenium and perovskite solar cells. As known, the Sb<sub>2</sub>Se<sub>3</sub> solar cell performance is closely related to the light absorber layer (crystallinity, composition and bulk defect density, etc.), PN heterojunction quality (charge carrier concertation, energy band alignment and interface defect density, etc.) and back-contact barrier formation, which determines the process of carrier generation, excitation, relaxation, transfer and recombination. The low fill factor is one of the core problems that limit further efficiency improvement of Sb<sub>2</sub>Se<sub>3</sub> solar cells, which can be attributed to the high potential barrier at the back contact between the Mo electrode and Sb<sub>2</sub>Se<sub>3</sub>absorption layer. In this work, a heat treatment is applied to the Mo electrode to generate a MoO<sub>2</sub>buffer layer. It can be found that this buffer layer can inhibit MoSe<sub>2</sub> film growth, exhibiting better ohmic contact with Sb<sub>2</sub>Se<sub>3</sub> and reducing the back contact barrier of the solar cell. Sb<sub>2</sub>Se<sub>3</sub> thin film is prepared by an effective combination reaction involving sputtered and selenized Sb precursor. After introducing the MoO<sub>2</sub>buffer layer, it can also promote the formation of (hk1) (including (211), (221) and (002) etc.) preferentially oriented Sb<sub>2</sub>Se<sub>3</sub> thin films with average grain size over 1 μm. And the ratio of Sb and Se was optimized from 0.57 to 0.62, approaching the stoichiometric ratio of Sb<sub>2</sub>Se<sub>3</sub> thin film and inhibiting the formation of <em>V<sub>se</sub> </em>and <em>Sb<sub>Se</sub></em> defects. Finally, it enhances the open-circuit voltage (<em>V<sub>OC</sub></em>) of solar cells from 0.473 V to 0.502 V, short-circuit current density (<em>J<sub>SC</sub></em>) from 22.71 mA/cm<sup>2</sup> to 24.98 mA/cm<sup>2</sup>, and fill factor (<em>FF</em>) from 46.90% to 56.18%, establishing a promotion in power conversion efficiency (<em>PCE</em>) from 5.04% to 7.05%. This work proposes a facile strategy for interfacial treatment and elucidates the related carrier transport enhancement mechanism, paving a bright avenue to overcome the efficiency bottleneck of Sb<sub>2</sub>Se<sub>3</sub> thin film solar cells.

Surface-Anchored Acetylcholine Regulates Band-Edge States and Suppresses Ion Migration in a 21%-Efficient Quadruple-Cation Perovskite Solar Cell.

Although incorporating multiple halogen (bromine) anions and alkali (rubidium) cations can improve the open-circuit voltage (Voc ) of perovskite solar cells (PSCs), severe voltage loss and poor stability have remained pivotal limitations to their further commercialization. In this study, acetylcholine (ACh+ ) is anchored to the surface of a quadruple-cation perovskite to provide additional electron states near the valence band maximum of the perovskite surface, thereby enhancing the band alignment and minimizing the Voc loss significantly. Moreover, the quaternary ammonium and carbonyl units of ACh+ passivate the antisite and vacancy defects of the organic/inorganic hybrid perovskite. Because of strong interactions between ACh+ and the perovskite, the formation of lead clusters and the migration of halogen anions in the perovskite film are suppressed. As a result, the device prepared with ACh+ post-treatment delivers a power conversion efficiency (PCE) (21.56%) and a value of Voc (1.21V) that are much higher than those of the pristine device, along with a twofold decrease in the hysteresis index. After storage for 720 h in humid air, the device subjected to ACh+ treatment maintained 70% of its initial PCE. Thus, post-treatment with ACh+ appears to be a useful strategy for preparing efficient and stable PSCs.

Effect of Ion Vacancy Migration on Open‐Circuit Voltage of Perovskite Solar Cells

Herein, the influence of ion vacancy migration on the open‐circuit voltage (Voc) of perovskite solar cells (PSCs) by computer simulation is explored. It is found that for PSCs, Voc is determined not only by the carrier recombination process, but also dominantly by the space charge effect of the ion vacancy stored within the Debye layers of the perovskite absorber layer. The migration of ion vacancy affects the ionic charge accumulation within the Debye layers, which affects the screen voltage via the space charge effect and thus determines Vocr and Vocf (open‐circuit voltage for reverse sweep and forward sweep, respectively). By tuning the parameters that determine ion vacancy mobility, the effects of ion vacancy mobility on Vocr and Vocf is investigated.

A comprehensive investigation of the potential of metal assisted chemical etched (MACE) nano-textures over conventional micron-sized iso-textures for industrial silicon solar cell applications

We present a comprehensive comparative analysis of acid textured and MACE (Metal Assisted Chemical Etching) nano-textured multi crystalline silicon (mc-Si) solar cells processed in an industrial cell line in terms of performance parameters. The batch average open circuit voltage of nano-textured solar cells was marginally lower and the measures for improving the same are discussed. Batch average short circuit current (JSC) and fill factor (FF) were respectively 0.65 mA-cm−2 and 0.7% (absolute) higher for MACE nano-textured solar cells. Significantly lower surface reflectance in MACE nano-textured cells in the blue and near infrared regions are found to be the reason for the enhancement in JSC. The main component of FF gain is identified as the lower series resistance in MACE cells. The contact formation mechanism in screen printed mc-Si solar cells, investigated using scanning electron microscopy, revealed that the elevated portions at the boundaries of the textures act as favorable sites for silver crystallite precipitation. A larger areal density of such sites is observed on the MACE nano-textured surface than in case of acid textured solar cells, leading to a much lower series resistance. The results suggest the potential for the application of MACE in the more popular mono-crystalline silicon cell technology for further reduction of contact resistance. Alternatively, MACE may be used to obtain similar contact resistance with lower amount of silver paste, compared to iso- and pyramid textured solar cells.

Application of bromide-iodide lead perovskite thin film as a copper-free back contact layer for CdTe solar cells

In this study, bromide-iodide lead perovskite (CH3NH3Pb(I1−xBrx)3) thin films were fabricated and applied as copper-free back contact layers for CdTe solar cells. The results reveal that the open-circuit voltage (Voc) and fill factor (FF) of the CdTe solar cells are greatly enhanced by the utilization of perovskite back contact layers. This is mainly due to the evidently reduced charge transportation barrier at the back contact, which is verified by temperature-dependent current–voltage (J-V-T) and apparent quantum efficiency (AQE) measurements. Specifically, after the application of perovskite back contact layer, the best-performing device shows 15.80% improvement in efficiency (from 10.82 to 12.53%), with Voc and FF increasing by 3.94% and 6.91%, respectively. Besides, the overall uniformity of the solar cells is also improved by the perovskite back contact, suggested by laser beam induced current (LBIC) results. Further simulation results confirm the experimental results and clarify the optimized cell performance by perovskite back contact layer in terms of both energy band alignment and charge carriers’ recombination. In addition, in the simulation section, we have also investigated the effect of the bandgap, thickness and doping level of perovskite and the doping level of CdTe on the cell performances. The experiments and the numerical simulation reported in this work suggest perovskite a potential copper-free back contact layer for the fabrication of efficient CdTe solar cells.

Sinc and discrete singular convolution for analysis of three‐layer composite of perovskite solar cell

This work studies the performance of composite solar cell model consisting of perovskite absorber layer (CH3NH3PbI3), electron transport layer (TiO2 or PCBM), and hole transport layer (Spiro-OMeTAD or CuI) via sinc and discrete singular convolution quadrature techniques. The governing equations for hole transport layer, electron transport layer, and absorber layer (perovskite) are derived depending on the equations of continuity and Poisson. Different quadrature and block-marching techniques convert these equations to the nonlinear algebraic system. Then, we apply the iterative method to solve the problem of nonlinearity. For each scheme, a programmed code is designed to get a numerical solution for composite perovskite solar cells by MATLAB. To ensure the efficiency and convergence of these schemes, the obtained results match with previous experiments and other numerical schemes. From these comparisons, the discrete singular convolution quadrature technique gives high accuracy, speed of convergence, and more reliability than other methods with error up to 10−8. Consequently, the effects of different thicknesses, mobilities, band gaps, temperatures, absorption factors, wavelengths, and doping concentrations on current density, open-circuit voltage, and efficiency of solar cells are investigated in a full parametric study. Hence, the obtained effects of the current schemes increase the efficiency of perovskite solar cells. Novelty Statement Sinc and discrete singular convolution quadrature techniques are applied to reach high performance for perovskite absorber layer sandwiched between electron transport layer (ETL) and hole transport layer (HTL). The study demonstrates that the discrete singular convolution quadrature method gives high efficiency and accurate results at different parameters. It is obtained that the power conversion efficiency is approximately 35% at specific parameters.

Charge transfer rates and electron trapping at buried interfaces of perovskite solar cells

<h2>Summary</h2> Identification of electronic processes at buried interfaces of charge-selective contacts is crucial for photovoltaic and photocatalysis research. Here, transient surface photovoltage (SPV) is used to study the passivation of different hole-selective carbazole-based SAMs. It is shown that transient SPV and transient photoluminescence provide complementary information on charge transfer kinetics and trapping/de-trapping mechanisms, and that trap-assisted non-radiative recombination losses originate from electron trapping at the SAM-modified ITO/perovskite interface. The hole transfer rates and the density of interface electron traps, obtained by fitting SPV transients with a minimalistic kinetic model, depended strongly on the SAM's chemical structure, and densities of interface traps as low as 10⁹ cm⁻², on par with highly passivated c-Si surfaces, were reached for Me-4PACz, previously used in record perovskite/silicon tandem solar cells. The extracted hole transfer rate constants and interface trap densities correlated well with the corresponding fill factors and open-circuit voltages of high-efficiency solar cells.

Nicotinamide‐Modified PEDOT:PSS for High Performance Indoor and Outdoor Tin Perovskite Photovoltaics

Tin perovskites are generally considered as promising candidates to solve the issue of toxicity for lead perovskites. However, the open‐circuit voltage (VOC) of tin perovskite solar cells is usually lower than expected. Herein, the hole‐transporting layer of PEDOT:PSS is treated with nicotinamide, and the tin perovskite active layer is deposited on the treated film to prepare a perovskite photovoltaic device. The fabricated device presented an increased VOC from 0.63 to 0.82 V, yielding an improved power conversion efficiency (PCE) increased from 4.77% to 8.28%. More importantly, under the weak light (1000 lux) irradiation of a simulated indoor light source, the device presented a PCE as high as 14.40%, which is the highest indoor efficiency for tin‐based perovskite photovoltaics reported so far. The promising indoor performance of tin perovskite photovoltaics paves a new way to fabricate eco‐friendly indoor photovoltaics by using lead‐free tin perovskites.

Near surface defects: Cause of deficit between internal and external open‐circuit voltage in solar cells

AbstractInterface recombination in a complex multilayered thin‐film solar structure causes a disparity between the internal open‐circuit voltage (VOC,in), measured by photoluminescence, and the external open‐circuit voltage (VOC,ex), that is, a VOC deficit. Aspirations to reach higher VOC,ex values require a comprehensive knowledge of the connection between VOC deficit and interface recombination. Here, a near‐surface defect model is developed for copper indium di‐selenide solar cells grown under Cu‐excess conditions. These cell show the typical signatures of interface recombination: a strong disparity between VOC,in and VOC,ex, and extrapolation of the temperature dependent q·VOC,ex to a value below the bandgap energy. Yet, these cells do not suffer from reduced interface bandgap or from Fermi‐level pinning. The model presented is based on experimental analysis of admittance and deep‐level transient spectroscopy, which show the signature of an acceptor defect. Numerical simulations using the near‐surface defects model show the signatures of interface recombination without the need for a reduced interface bandgap or Fermi‐level pinning. These findings demonstrate that the VOC,in measurements alone can be inconclusive and might conceal the information on interface recombination pathways, establishing the need for complementary techniques like temperature dependent current–voltage measurements to identify the cause of interface recombination in the devices.

Slow Relaxation of Photogenerated Charge Carriers Boosts Open-Circuit Voltage of Organic Solar Cells.

Among the parameters determining the efficiency of an organic solar cell, the open-circuit voltage (VOC) is the one with most room for improvement. Existing models for the description of VOC assume that photogenerated charge carriers are thermalized. Here, we demonstrate that quasi-equilibrium concepts cannot fully describe VOC of disordered organic devices. For two representative donor:acceptor blends, it is shown that VOC is actually 0.1–0.2 V higher than it would be if the system was in thermodynamic equilibrium. Extensive numerical modeling reveals that the excess energy is mainly due to incomplete relaxation in the disorder-broadened density of states. These findings indicate that organic solar cells work as nonequilibrium devices, in which part of the photon excess energy is harvested in the form of an enhanced VOC.

Regulation of selenium composition by supercritical carbon dioxide for CZTSSe solar cells efficiency improvement

Se-rich selenization offers remarkable benefits for grain growth of Cu2ZnSn(S,Se)4 (CZTSSe) absorber layers, which is critical for solar cells fabrication. However, low-activity selenium often forms Se-clusters and trapped in CZTSSe absorber layers. These trapped Se-clusters increase carrier recombination in CZTSSe absorber layers, thus decreasing efficiency of CZTSSe solar cells. Herein, supercritical carbon dioxide (SCCO2) treatment was demonstrated to regulate selenium composition of CZTSSe absorber layers for solar cells efficiency improvement. Compared with untreated CZTSSe absorber layers, selenium composition of CZTSSe absorber layers were significantly reduced by SCCO2 treatment. The open circuit voltage (VOC) and fill factor (FF) of CZTSSe solar cells improved pronouncedly due to carrier recombination reduction and carrier transport enhancement. As a consequence, the power conversion efficiency (PCE) was increased from 9.26 % (ref device) to 11.43 % (SCCO2 device). This unique SCCO2 treatment may offer new ideas for other selenization-based optoelectronic devices performance improvement.

Hydroxyl‐Rich d‐Sorbitol to Address Transport Layer/Perovskite Interfacial Issues toward Highly Efficient and Stable 2D/3D Tin‐Based Perovskite Solar Cells

Abstract2D/3D mixed tin (Sn)‐based perovskites serving as excellent active layers possess a potential for enhancing film/device stabilities, open‐circuit voltage, and power conversion efficiency (PCE) of perovskite solar cells (PSCs). Unfortunately, the poor morphology and low film coverage of these 2D/3D perovskites jeopardize their device performance and stability. Herein, doping of hydroxyl‐rich d‐sorbitol into hole transport layer poly(3,4‐ethenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) can effectively address the PEDOT:PSS/perovskite interfacial issues. The multihydroxyl groups from d‐sorbitol can sufficiently anchor iodide ions ([SnI6]4− or free I−) of PEA0.1(FA0.75MA0.25)0.9SnI3 perovskite via hydrogen bond, which not only introduces more nucleation sites with improved crystallinity along the preferential crystal orientations, but also reduces the trap states’ density and suppresses the iodide ions’ migration. In the meanwhile, the content of SnI2 is reduced in the resulting perovskite films. All of these are conducive to form compact and smooth high‐quality perovskite films with less traps and enhanced stability. Benefitting from these, an inverted PSC with a champion PCE of 10.46% and a lifetime of 700 h (25% attenuation in N2) is eventually achieved. This work provides a promising approach to manufacture highly efficient and stable Sn‐based PSCs based on 2D/3D mixed perovskites.

Improved open-circuit voltage of CsPbI3 quantum dot solar cells by PMMA interlayer

All-inorganic CsPbI3 quantum dots perovskite solar cells (QDPSCs) have wide potential application due to their excellent photoelectric properties with high environmental stability. However, the poor interface contact between the light absorption layer and hole transport layer (HTL) results in the serious carrier recombination and the large open-circuit voltage (VOC) loss. Herein, we introduce the insulating PMMA (poly(methyl methacrylate)) buffer layer into CsPbI3 quantum dots (QDs)/Cu12Sb4S13 QDs HTL interface in all-inorganic QDPSCs with the structure of FTO/c-TiO2/m-TiO2/CsPbI3 QDs/Cu12Sb4S13 QDs/Au. The interface carrier recombination is obviously suppressed and the significant VOC improvement from 1.04 V to 1.14 V is obtained with the incorporated PMMA buffer layer. The devices with PMMA buffer layer show the enhanced power conversion efficiency (PCE) of 10.99%, and the improvement of 9.71% is achieved compared to that of the control devices (10.02%). The stability is significantly enhanced due to the intrinsic hydrophobic characteristic of PMMA, and the devices retain 62.69% of their initial PCE after stored in air condition for 15 days, which is greatly improved compared with the control device without PMMA.

Simulation and characterization of planar high-efficiency back contact silicon solar cells

Short-circuit current, open-circuit voltage, and photoconversion efficiency of silicon high-efficiency solar cells with all back contact (BCSC) with planar surfaces have been calculated theoretically. In addition to the recombination channels usually considered in this kind of modeling, namely, radiative, Auger, Shockley–Read–Hall, and surface recombination, the model also takes into account the nonradiative trap-assisted exciton Auger recombination and recombination in the space charge region. It is ascertained that these two recombination mechanisms are essential in BCSCs in the maximum power operation regime. The model results are in good agreement with the experimental results from the literature.

Passivation of the Buried Interface via Preferential Crystallization of 2D Perovskite on Metal Oxide Transport Layers.

The open-circuit voltage (Voc ) of perovskite solar cells is limited by non-radiative recombination at perovskite/carrier transport layer (CTL) interfaces. 2D perovskite post-treatments offer a means to passivate the top interface; whereas, accessing and passivating the buried interface underneath the perovskite film requires new material synthesis strategies. It is posited that perovskite ink containing species that bind strongly to substrates can spontaneously form a passivating layer with the bottom CTL. The concept using organic spacer cations with rich NH2 groups is implemented, where readily available hydrogens have large binding affinity to under-coordinated oxygens on the metal oxide substrate surface, inducing preferential crystallization of a thin 2D layer at the buried interface. The passivation effect of this 2D layer is examined using steady-state and time-resolved photoluminescence spectroscopy: the 2D interlayer suppresses non-radiative recombination at the buried perovskite/CTL interface, leading to a 72% reduction in surface recombination velocity. This strategy enables a 65mV increase in Voc for NiOx based p-i-n devices, and a 100mV increase in Voc for SnO2 -based n-i-p devices. Inverted solar cells with 20.1% power conversion efficiency (PCE) for 1.70eV and 22.9% PCE for 1.55eV bandgap perovskites are demonstrated.

Self-Induced Type-I Band Alignment at Surface Grain Boundaries for Highly Efficient and Stable Perovskite Solar Cells.

The open-circuit voltage (VOC ) of perovskite solar cells (PSCs) is reported to be significantly weakened by carrier loss at the film surface. Here, the moisture condensation at only the upper surface of perovskite films is controlled by constructing an ultralow surface temperature. Then, type-I band alignment can be formed at the surface grain boundaries due to the etching effect of trace amounts of condensed moisture. The beneficially constructed surface type-I band alignment can effectively repel carriers and return them to the inside of the grain, significantly avoiding the carrier loss at films surface. As a result, a superior carrier lifetime exceeding 2.5µs is obtained and the VOC of PSC is remarkably boosted from 1.07 to 1.17V. The minimum VOC deficit of only 0.39V enables a substantial gain in power conversion efficiency (PCE) from 20.2% to 22.4% in one-step spin-coating methods. Moreover, this innovation is versatile and a champion PCE of 23.2% is also achieved in two-step spin-coating methods.

From Frequency Domain to Time Transient Methods for Halide Perovskite Solar Cells: The Connections of IMPS, IMVS, TPC, and TPV.

The correlation of different methods of measurement can become an important tool to identify the dominant physical elements that govern the electronic and ionic dynamics in perovskite solar cells. The diverse phenomena underlying the response of halide perovskite materials to different stimuli are reflected in time-domain measurements, where transients appear with time scales spanning orders of magnitude, from nanoseconds to hours. We discuss the connection between different frequency- and time-domain methods to probe the voltage and current response of halide perovskite solar cells to different small perturbations. To solve the frequency-to-time transformation, we start from models of the transfer function of intensity-modulated photocurrent spectroscopy (IMPS) and derive the associated impulse response function, the transient photocurrent (TPC), in response to a short light pulse. Similarly, we determine the transient photovoltage (TPV) starting from the intensity-modulated photovoltage spectroscopy (IMVS) transfer function. We also discuss the open-circuit voltage decays (OCVD). We first show the response of simple equivalent circuit models, and then we treat the full model for generation–diffusion–recombination of electrons that shows a spiraling loop in IMPS. This model gives rise to overshoots in the time domain.

Evaluation of a wireless low-energy mote with fuzzy algorithms and neural networks for remote environmental monitoring

The devices developed for applications in the internet of things have evolved technologically in the improvement of hardware and software components, in the area of optimization of the life time and to increase the capacity to save energy. This paper shows the development of a fuzzy logic algorithm and a power propagation neural network algorithm in a wireless mote (IoT end device). The fuzzy algorithm changes the transmission frequency according to the battery voltage and solar cell voltage. Moreover,the implementation of algorithms based on neural networks, implied a challenge in the evaluation and study of the energy commitment for the implementation of the algorithm, memory space optimization and low energy consumption.

Energetics and Kinetics Requirements for Organic Solar Cells to Break the 20% Power Conversion Efficiency Barrier

The thermodynamic limit for the efficiency of solar cells is predominantly defined by the energy bandgap of the used semiconductor. In case of organic solar cells both energetics and kinetics of three different species play role: excitons, charge transfer states and charge separated states. In this work, we clarify the effect of the relative energetics and kinetics of these species on the recombination and generation dynamics. Making use of detailed balance, we develop an analytical framework describing how the intricate interplay between the different species influence the photocurrent generation, the recombination, and the open-circuit voltage in organic solar cells. Furthermore, we clarify the essential requirements for equilibrium between excitons, CT states and charge carriers to occur. Finally, we find that the photovoltaic parameters are not only determined by the relative energy level between the different states but also by the kinetic rate constants. These findings provide vital insights into the operation of state-of-art non-fullerene organic solar cells with low offsets.

How band tail recombination influences the open‐circuit voltage of solar cells

AbstractThe power conversion efficiency of solar cells strongly depends on the open‐circuit voltage which, in turn, depends on the recombination activity within the device. A possible source of detrimental charge carrier recombination is band tails. An empirical linear relationship between loss and the Urbach energy of the band tails has been shown in the past. Here we discuss how band tails influence the radiative recombination and the nonradiative recombination in the bulk of the absorber. First, we show through photoluminescence that the band tails can be willfully tuned in state‐of‐the‐art thin‐film Cu (In,Ga)Se2 (CIGSe) absorbers and solar cells on a 20% efficiency level and beyond through the incorporation of alkali atoms. In the second part, we compare our CIGSe results to published results from other solar cell technologies. This comparison reveals that CIGS solar cells follow the previously described empirical trend: an increase in the open‐circuit voltage with decreasing band tails. Finally, we model the influence of tail states on the radiative and nonradiative recombination losses: Radiative recombination is increased because carriers thermalize into the tail states and nonradiative recombination of free carriers in the bands is increased because of Shockley–Read–Hall recombination through the tail states. The comparison with experimental data shows that the influence of tail states is even worse than the increase in radiative and SRH recombination predicted by our model. Our results thus suggest that band tails act as one of the main remaining voltage limitations in the majority of state‐of‐the‐art solar cells.